DE 102 08 286 A1 discloses an electronic image evaluation device and an evaluation method in which a printed product is conveyed past the device and, during its movement, images of predetermined extracts of the printed product are acquired and evaluated. In order to trigger the image acquisition at the respectively correct time, a sensor is provided which outputs a synchronization signal when it detects a predetermined reference marking on the printed product. The extracts of the printed product to be acquired have known positions in relation to the reference marking, so that the correct triggering time for the image acquisition can be determined by using the synchronization signal and the known speed of the printed product. However, the aforementioned specification contains no more specific statements relating to the implementation and internal functioning of the sensor in question.
A typical example of an extract of a printed product to be acquired optically and evaluated within a press is a control strip arranged outside the subject and printed with test patterns. Such control strips, whose longitudinal direction is normally transverse to the transport direction of the printing material, contain a set of measuring areas that are repeated periodically in the aforesaid longitudinal direction, and on each measuring area a specific characteristic variable characterizing the printing quality can be measured. One example of the configuration and use of such a control strip is given by DE 195 38 811 C2. In order to minimize the consumption of printing material, attempts are made to keep control strips of this type as narrow as possible. Consequently, the area required for the reference marking needed for the synchronization of the image position with the movement of printing material, which will be designated the “trigger mark” in the further text, should also be as small as possible.
On the other hand, during the movement of the printing material in a press, position tolerances in the lateral direction must be expected. In particular, in the case of work-fed rotary machines, the lateral deviation of the printing material web from its intended position within the aforesaid web length in the machine can amount to some centimetres. In order to ensure that a trigger mark can be registered by a sensor arranged at a specific lateral position even in the least favourable case of a lateral position or deviation of the printing material web, a trigger mark would therefore have to have a considerable width covering the entire tolerance range. However, this is inconsistent with the endeavour previously mentioned to have the smallest possible area required for the pattern to be provided outside the subject merely for control purposes.